Synthetic lubricants containing trihydroxydiphenyl



United States Patent 3,326,802 SYNTHETIC LUEREQANTS CUNTAENING TRlHYDROXt DWHENYL Joseph .l. McGrath, Monroevilie, and Harold 0. Strange, Penn Hiils Township, Allegheny fiounty, Pa, assignors to Gulf Research 8; Development t'lompany, Pittsburgh, Pin, a corporation of Delaware No Drawing. Filed Feb. ll. ices, Ser. No. 523,835 4 Claims. (Cl. 252-56) This invention relates to a lubricating oil composition and more particularly to a synthetic ester lubricant which is highly resistant to oxidational changes at an elevated temperature for prolonged periods of time.

The current trend in designing modern aircraft engines has accentuated the need for lubricating compositions which will elfectively lubricate bearings operating at high rotational speeds at high as well as low temperatures. It is not uncommon for present aircraft lubricants to encounter temperatures ranging from below 65 F. to above 300 F. Thus, aircraft lubricants must possess high viscosity indices. In addition to high viscosity indices, aircraft lubricants should have low evaporation properties, high dash and tire points and should be highly resistant to oxidative deterioration. Products which are formed upon oxidation of an oil are acidic in nature and thus exert a corrosive efi'ect on metal surfaces with which the oil comes in contact. Oxidation of the oil is further undesirable in that upon oxidation its viscosity is changed. Still further, oxidation of the oil gives rise to the formation of gum, varnish, sludge and other deleterious products which decrease the useful life of the oil. Although excellent progress has been made in petroleum technology through refining techniques and the utilization of oil additives, mineral lubricating oils and compounded mineral oils have not been completely satisfactory under the severe conditions encountered in aircraft engines, particularly the newer turbojet and turboprop engines.

Some synthetic lubricating oils have been produced which have high viscosity indices, low evaporation properties, high flash and fire points, and are substantially resistant to oxidational changes at temperatures up to about 300 F. For example, certain organic esters including monoesters of monobasic acids, diesters of dibasic acids, triesters of tribasic acids, diesters prepared from glycol, triesters prepared from polyhydric alcohols and mixtures thereof have a majority of the properties required of aircraft lubricants and have become commercially important. Specific examples of the ester lubricants include di-(3,5,5-trimethylhexyl)glutarate, di-(2-ethylhexyl)adipate, diisooctyl adipate, diisodecyl adipate, di-(Z- ethylhexyl)pimelate, dihexyl azelate, diisooctyl azelate, di-(Z-ethylhexyl) azelate, di-(Z-ethylhexyl) sebacate and diproplyene glycol dipelargonate. While the ester lubricants in general have been satisfactory at temperatures up to about 300 F., these lubricants have not been completely satisfactory when used for prolonged periods of time under oxidizing conditions at temperatures above about 300 F. Even in those instances where ester lubricants are not sufiiciently resistant to oxidational deterioration at temperatures below about 300 F., their stabilization at low temperatures has been effected by the use of a conventional mineral oil oxidation inhibitor. However, at temperatures above about 300 R, such as those temperatures encountered in the lubrication of turbojet and turboprop engines, the use of a conventional mineral oil oxidation inhibitor does not give satisfactory stabilization for prolonged periods of time.

We have discovered that a synthetic ester lubricating composition having improved thermal stability for an extended period of time when used to lubricate bearings at temperatures above about 300 F. under oxidizing conditions can be obtained by incorporating trihydroxydiphenyl into the synthetic ester lubricant in oxidationinhibiting proportions. Thus, the improved lubricating composition of our invention comprises a synthetic ester lubricant normally tending to undergo oxidational changes at an elevated temperature above about 300 F. and a small amount, sufficient to substantially retard such oxidational changes, of trihydroxydiphenyl.

Trihydroxydiphenyl is available commercially so that neither the compound per se nor its method of preparation constitutes any portion of the invention. Trihydroxydiphenyl can exist as a number of isomers any one of which or mixture thereof can be employed in accordance with the invention. Because of the difficulty encountered in obtaining individual isomers of trihydroxydiphenyl, a m'xture of isomers of trihydroxydiphenyl is preferred for economic reasons. Commercially available mixtures of trihydroxydiphenyl may contain minor amounts of impurities including dihydroxydiphenyl and resorcinol without seriously modifying the beneficial properties imparted to the lubricant by the trihydroxydiphenyl. However, We prefer to employ commercially available trihydroxydiphenyl concentrates which have been recrystallized at least once from water.

The amount of the trihydroxydiphenyl employed in the lubricating composition of our invention depends upon the particular ester lubricant used as Well as the severity of the conditions to which it is subjected. Normally, the amount of the trihydroxydiphenyl comprises about 0.1 to about 5 percent by weight based on the weight of the total composition. In most instances, good results are obtained with about 0.5 to about 2 percent by Weight. In any event a small amount, suificient to inhibit oxidational changes normally tending to occur, is used.

The synthetic ester lubricant to which the trihydroxydiphenyl is added according to this invention can be a monoester of a monobasic acid, a diester of a dibasic acid, a triester of a tribasic acid, a glycol diester, a polyhydric alcohol triester, or a mixture of esters having a majority of the properties of a mineral oil of lubricating grade. In addition to possessing a majority of the properties of a mineral lubricating oil, the ester lubricants, at least initially should be substantially neutral.

In a preferred embodiment of the invention, we utilize a substantially neutral ester of a dibasic acid containing 2 to 10 carbon atoms and an aliphatic alcohol containing 2 to 18 carbon atoms as the synthetic lubricating oil. Diesters of aliphatic dibasic acids containing 6 to 10 carbon atoms and alcohols containing at least four and preferably between 6 and 16 carbon atoms are particularly desirable.

Specific examples of some of the alcohols which can be used in preparing the ester lubricants are l-butanol; Z-butanol; Z-methyl-Z-propanol; l-pentanol; 2-pentanol; Z-methyl-Z-butanol; l-hexanol; Z-hexanol; 3-hexanol; 2- methyll -pentanol; 3-methyl-l -pentanol; 4-methyl l -pentanol; 2,4-dimethyl-2-pentanol; 2,3-dimethyl-3-pentanol; 2,4-dimethyl-3-pentanol; 3-ethyl-3-pentanol; Z-methyl-lhexanol; S-methyl-l-hexanol; Z-methyl-Z-hexanol; 5- methyl-Z-hexanol; 3-methyl-3-hexanol; 5-methy1-3-hexanol; l-heptanol; Z-heptanol; 4-heptanol; 2-methyl-2-heptanol; 3-methyl-2-heptanol; 4-methyl-4-heptanol; 2-ethy1- butanetriol; 1,2,6-hexanetriol; 1,4,7-octanetriol; and the like, as well as mixtures of two or more of such alcohols.

Specific examples of some of the acids with which the above-enumerated alcohols can be reacted in preparing the esters for use in the compositions of the present invention are oxalic, malonic, succinic, isosuccinic, glutaric, ethyl malonic, pyrotartaric, adipic, pimelic, suberic, azelaic, sebacic, phthalic, aconitic and tricarballylic acid. When a low molecular weight acid is esterified, a high molecular weight alcohol is preferred in order to produce an ester having a majority of the properties of a mineral oil of lubricating grade. While the esters of the aliphatic dibasic acids are preferred, the esters of aromatic dibasic acids such as the phthalic acid ester of a material such as castor oil or other high molecular weight alcohols can also be used.

Examples of preferred synthetic diester lubricants to which the trihydroxydiphenyl is added according to the invention are the substantially neutral esters of hexyl, octyl, decyl, lauryl, myristic and cetyl alcohols and adipic, pimelic, suberic, azelaic and sebacic acids.

Specific examples of especially effective diand triester lubricants are diisooctyl adipate; diisodecyl adipate; di-2-ethylhexyl azelate; di-2ethylhexyl sebacate; diisooctyl azelate; diisooctyl sebacate; glycerol trihexanoate; 1,4,7-octanetriol tributanoate; trimethylolethane trihexanoate; trimethylolpropane trihexanoate; trimethylolpropane triheptanoate; triamyl aconitate; trihexyl tricarballylate; and monoethyl dioctyl tricarballylate.

The particular synthetic ester lubricant employed, as well as the exact amount of such lubricant, depends upon the characteristics desired in the final lubricating composition. In most instances, however, when the synthetic lubricant comprises an ester of an aliphatic dibasic acid, an ester of a tribasic acid or a triester of a polyhydric alcohol, the ester comprises at least about 95 percent by weight of the total composition.

The synthetic lubricating oil composition of this invention can contain other addition agents normally added to oils for a specific purpose, if desired, such as an oiliness and extreme pressure agent, a corrosion inhibitor, a foam suppressant, a sludge inhibitor, a viscosity index improver, a thickener, a pour point depressant, a dye, and the like.

In order to demonstrate the effectiveness of trihydroxydiphenyl as an antioxidant in a synthetic ester lubricant, we have conducted comparative oxidation tests with a representative synthetic ester lubricant alone and with the same lubricant blended with a commercial trihydroxydiphenyl concentrate and with recrystallized trihydroxydiphenyl. In brief, the comparative tests were conducted by bubbling air at a rate of one liter per hour through approximately 20 milliliters of test lubricant contained in a glass tube inserted into an aluminum block heated to 347 (175 C.). The duration of the test was 48 hours. The viscosity of the test lubricant and its acid number were determined at the beginning and the conclusion of each test. The percent increase in viscosity and the increase in acid number of the lubricant during the test is indicative of the oxidation stability of the test lubricant at a temperature of 347 F. At the end of each test an observation was also made as to whether or not varnish and/or sediment were present.

The results of comparative tests on the lubricant alone and the lubricant containing commercial trihydroxydiphenyl and recrystallized trihydroxydiphenyl are shown in Table I. The commercial trihydroxydiphenyl was a yellowish-red resinous-like material having no sharp melting point but became soft and less viscous as the temperature was increased. The commercial trihydroxydiphenyl which was recrystallized from water comprised yellowish crystals melting at about 5565 C.

TABLE I Composition, percent by weight:

Lubricating Oil: Diisooctyl azelate Trihydroxydiphenyl:

Trihydroxydiphenyl (concentrate) Triliydroxydiphenyl (recrystallized) Oxidation Test (Air Flow: 1 l./hr.)

Viscosity, cs. at F.:

Visual Observation 1 Clear, yellow. 2 Clear, red.

The data in Table I clearly show that the oxidation stability of diisooctyl azelate at a temperature of 347 F. can be greatly improved by the addition of either a commercially available trihydroxydiphenyl concentrate or recrystallized trihydroxydiphenyl.

While our invention has been described above with reference to various specific examples and embodiments, it will be understood that the invention is not limited to such examples and embodiments and may be variously practiced within the scope of the claims hereinafter made.

We claim:

1. A lubricating composition comprising a major amount of a synthetic ester lubricant normally tending to undergo oxidational changes at an elevated temperature and a minor amount, sufficient to substantially retard such oxidational changes, of trihydroxydiphenyl.

2. The lubricating composition of claim 1 wherein the amount of the trihydroxydiphenyl is about 0.1 to about 5 percent by weight of the composition.

3. The lubricating composition of claim 1 wherein the synthetic ester lubricant is diisooctyl azelate.

4. The lubricating composition of claim 3 wherein the amount of the trihydroxydiphenyl is about 0.5 to about 2 percent by weight of the composition.

References Cited UNITED STATES PATENTS 1,993,771 3/1935 Calcott et al 252-404 2,086,418 7/1937 Hunt et al 252404 2,324,186 7/ 1943 Armstrong et al. 252-404 2,479,948 8/ 1949 Luten et al. 260-620 2,697,111 12/1954 Bell et al. 260-620 2,704,773 3/1955 Young et al. 2 60-620 3,218,256 11/1965 Edwards et al 252-475 FOREIGN PATENTS 584,611 11/195 8 Italy.

DANIEL E. WYMAN, Primary Examiner.

L. G. XILARHOS, Assistant Examiner, 

1. A LUBRICATING COMPOSITION COMPRISING A MAJOR AMOUNT OF A SYNTHETIC ESTER LUBRICANT NORMALLY TENDING TO UNDERGO OXIDATIONAL CHANGES AT AN ELEVATED TEMPERATURE AND A MINOR AMOUNT, SUFFICIENT TO SUBSTANTIALLY RETARD SUCH OXIDATIONAL CHANGES, OF TRIHYDROXYDIPHENYL.
 3. THE LUBRICATING COMPOSITION OF CLAIM 1 WHEREIN THE SYNTHETIC ESTER LUBRICANT IS DIISOOCTYL AZELATE. 